The forming of automotive body panels such as doors or decklids is accomplished through the interaction of an initially planar blank with complementary, contoured forming tools, or dies, under the urging of a load applied by a forming press. The load distribution on the dies is highly non-uniform, with some regions of the dies experiencing high loads while others are only lightly loaded. Because there will be at least some relative motion between the sheet metal blank and the dies, the magnitude of the local applied load significantly influences the local interaction between the blank and the dies.
Under high load, the interaction between sheet and die can lead to die wear which can modify the die surface finish and die geometry as well as to transfer of material from the blank to the die, usually described as galling or ‘die pick-up’. Galling and ‘die pick-up’ are different, but related mechanisms. The transfer of aluminum from a workpiece to a dies can lead to accumulation of material, resembling a particle on the surface of the die. This is ‘die pick-up’. Once a particle is present on the die surface, it can scratch or gall subsequent panels. Galling can lead to additional material transfer or pick-up.
These undesired outcomes have been managed by increasing the die hardness, frequently though surface coatings, or by application of lubricants or combinations of these approaches. However, such coatings and lubricants are expensive; frequently create housekeeping issues related to their application; and convey only temporary benefit thereby necessitating frequent re-application to sustain their effectiveness.
Thus there is a need for processes capable of imparting more durable lubricating surface to dies to better manage the sheet forming process.